Lotus Elan and Lotus Elan Plus 2 Forums

I have a couple heads in the machine shop on which the cams will not turn. My machine shop has a mandrel long enough and the right diameter to line hone but we can not find a housing bore dimension. Anyone have a number for me?

Surely that depends on the cams and whether or not the head needs sleeve bearings for the cams.

All decent cams have 1.000 inch journals and all lotus built heads that I know of require shell bearings. Thickness of the ones I am using for test are .0605 . ovbiously that gives about 1 1/8 for the size but I'd like to get down to a real number with a tolerance. Published clearance is .0005 to .002

My new head from SAS Engineering of Peterborough does not use CAM shells.

The Wilkins engine book quotes the "line bore in head" as 1.125-1.126 inch.
Brian

The normal reason cams do not turn is that the head has warped distorting the original line bore for the cams. The problem can be made worse by then machining the head face flat so it bolts down with distorted cam bores.

You need to understand the state of the head fully and the rectification may involve straightening the head to remove the cam line bore distortion and then machining the head face if needed rather than re line boring the cams bearings.

cheers
Rohan

Thanks ncm. Just what I needed

Thanks Rohan. I have had heads straightened in the past and even just bolted a warped one to a straight block. I am of the belief that once warpage happens heads want to stay that way. The straightened ones want to. Re warp so i am going for correction. Hope it works...

ncm wrote:The Wilkins engine book quotes the "line bore in head" as 1.125-1.126 inch.

Ford dimensions for the Twin Cam:

Cam journal diameter: 1.0000 to 1.0005"
Bearing clearance: 0.0005 to 0.002"

There's no dimension given for "line bore in head". I don't know where this has come from and personally I would treat Wilkins data with caution (at best it's already 2nd hand if from Ford and 3rd hand if from Lotus!). "line bore in head" does not take into effect bearing thickness and crush (which may vary depending on bearing manufacturer). It does not appear to be a manufacturer stated dimension. Just line bore to get the correct bearing clearance. You can measure each tunnel individually in any case. It the head is bent the individual tunnel dimensions should still be reasonably OK - it's only that they are no longer in line that causes the binding!

ACL have a specified housing fit for their cam bearings of 1.1255 to 1.1260 inch for the twin cam

cheers
Rohan

By a process of elimination, i.e. just fit 2 caps at a time, if you are lucky it’s just one cap that’s causing the problem. I have in the past hand scraped the high point, old school technique, to provide a free running cam.

Craven wrote:By a process of elimination, i.e. just fit 2 caps at a time, if you are lucky it’s just one cap that’s causing the problem. I have in the past hand scraped the high point, old school technique, to provide a free running cam.

Sorry but when you have hand scraped the metal away ok it will not jam . But what happens to the opposite side that has a gap and needs metal adding
I think the head will need to be stress relieved first then Cap mating faces machines a little. Then all Linebored to original Dias. Of course this will result in thinner Shims.
Alan

Progress report: The later of the two heads now allows the cams to turn.. We line honed just a whisker out of the bores and whatever warpage in the head was there is corrected. Bores cleaned at 1.126 so all good. Earlier head is more confusing. One bore is now ok but second side appears to have two mismatched caps. The odd thing is that they sppear to have matching factory stamped numbers on them. Anyone know if the numbers repeat on multiple heads??

Billmack wrote:Progress report:
...
One bore is now ok but second side appears to have two mismatched caps. The odd thing is that they sppear to have matching factory stamped numbers on them. Anyone know if the numbers repeat on multiple heads??

you mean head number is also stamped on each caps? or just the cap number 1 to 10...
in the latter case, someone with a couple heads taking them apart and mixing the caps would easily end up with 2 heads with non turning cams...

alan.barker wrote:

Craven wrote:By a process of elimination, i.e. just fit 2 caps at a time, if you are lucky it’s just one cap that’s causing the problem. I have in the past hand scraped the high point, old school technique, to provide a free running cam.

Sorry but when you have hand scraped the metal away ok it will not jam . But what happens to the opposite side that has a gap and needs metal adding
I think the head will need to be stress relieved first then Cap mating faces machines a little. Then all Linebored to original Dias. Of course this will result in thinner Shims.
Alan

So the clearance is increased by a couple of tenths so what!